Alkali pretreatment in sulfate process to prevent malodors



United States Patent ALKALI PRETREATMENT IN SULFATE PROCESS TO PREVENT MALODORS Erik Oman, Stocksund, Sweden, assignor to Skogsagarnas Industri Aktiebolag, Vaxjo, Sweden, a corporation of Sweden No Drawing. Continuation-impart of application Ser. No. 261,213, Feb. 26, 1963. This application May 23, 1967, Ser. No. 640,501

8 Claims. (Cl. 162-37) ABSTRACT OF THE DISCLOSURE CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 261,213, filed Feb. 26, 1963, and now abandoned.

BACKGROUND OF THE INVENTION (1) Field of the invention The invention pertains to the production of sulphate cellulose by digestion of cellulosic fiber material with alkali and sulphide, and more particularly to a novel pretreatment of the fiber material which prevents the formation of malodorous sulphurous substances in the subsequent sulphate digestion of the thus pre-treated fiber material.

(2) Description of the prior art It is known (a) that if cellulose fiber material is digested with NaOH alone (the plain alkali digestion process), no malodorous substances are formed. Wells et al., Kraft Pulping with Low Sulphidity, Paper Trade Journal, TAPPI Section, pages 103-106, Aug. 23, 1941, disclose (b) that a marked reduction in mercaptans and other substances causing objectionable odor is obtained by using a cooking liquor of low sulphidity. In neither of such prior processes (a) and (b), however, is there employed a conventional sulphate fiber digesting liquor, that is, one containing NaOH and Na s, nor is it possible to employ such conventional sulphate liquor while still retaining the advantages of the elimination of or the marked reduction in the amount of mercaptans and the like produced.

SUMMARY OF THE INVENTION The method of the invention includes a novel pretreatment of fiber material in combination with a subsequent digestion of the pre-treated material by a conventional sulphate digestion process. In the pre-treating of the fiber material it is preferably treated in an alkaline hydroxide solution having a concentration corresponding to 5-30 g./l. NaOH, at a temperature of from 30 to 90 7 C., and for a period of from 5 to 30 minutes. More specifically, in preferred embodiments the hydroxide may be sodium hydroxide, the solution is substantially free of sulphur and sulphur compounds, the alkaline liquor has a concentration of 6-10 g. alkali/l. solution at the be 3,397,109 Patented Aug. 13, 1968 ginning of the pre-treatment, the alkali in the pre-treatment solution is approximately the quantity required for digestion, the pre-treating liquor may be separated prior to the subsequent digestion process, and the pretreatment is carried out in a vessel separate from the sulphate digestion boiler. However, it is also possible to carry out the pre-treatment in a section of the sulphate digestion boiler. In a further embodiment of the invention said pre-treatment at least partly is carried out by means of black lye-obtained firom a preceding sulphate digestion of cellulosic material pre-treated with alkaline liquor simultaneously with filling wooden chips into a boiler for subsequent sulphate digestion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the production of sulphate-cellulose, wood (chips) is boiled in a digesting liquor containing NaOH and Na s and an admixed variable quantity of black liquor from a preceding digesting process. Hereby, however, a very nauseating, unpleasant odor arises during the digestion. The substances of bad odors principally consist of methyl mercaptane (CH SH), methyl sulphide (CH S as well as methyl disulphide (CH S The first-mentioned of these substances has a most intensive odor and is therefore most difi'icult to remove in a satisfactory manner.

An explanation of the formation of methyl mercaptane is apparent from the formula R.O.CH +NaSH- RONa+ CH SH Probably, both of the other substances are formed from the methyl mercaptane secondarily. R.O.CH is an ether and it is believed that it originates from the lignine-part of the wood. It is further believed that the ether decomposes during the digestion stage (when the temperature is increased), and that it results in CH SH and CH OH according to the following formulae:

ROCH +NaAH- RONa+ CH OH ROCH NaSH- RONa+CH SH If the digestion is carried out with NaOH only (the plain alkali digestion process), no substances having bad odors are formed, as known.

The present invention is substantially characterized in that the fiber material prior to the digestion process is soaked or in another manner pre-treated with a solution of alkaline liquor, for example alkali-hydrate or black lye, of such concentration and under such reaction conditions with respect to temperature and reaction-time, that the alkali is reacted with the CH groups of the cellulose-containing fiber material This prevents the formation of the malodorous compounds during subsequent sulphate digestion, since there are no CH -groups available for reaction with the sulphur containing compounds in the digestion vessel. Thus according to the invention it is suprisingly possible to avoid the formation of malodorous compounds in the digestion of cellulose according to the sulphate process. This surprising result has been fully proved by practical tests.

THE PRE-TREATMENT The wood to be pre-treated with sodium hydroxide lye can be of difierent kinds, such as common boiling chips, mechanically cut chips or special mechanically processed chips, so-called fiber-divided wood (also processed together with sodium lye). When the resin content of the wood is high, the resin is dissolved by the sodium-lye, 1 part by weight NaOH dissolving 7.6 parts by weight of resin acid. Instead of wood there can be used straw, grass, etc.

If the wood (the chips) is processed in an apparatus for producing so-called fiber-divided wood, it is suitable to carry out this processing with the sodium-lye, which is to be used in the pre-treatment, whereby both operations take place simultaneously.

The treatment of the wood (the chips) with the pretreating liquor, for example sodium-lye (beside the mechanical processing) can be carried out in many ways, of which the following can be stated:

(a) The chips are soaked in the pre-treating liquor and subsequently introduced into the boiler.

(b) The chips are sprayed with the pre-treating liquor during the filling of the chips into the boiler, whereafter the excess liquor is allowed to flow off.

(c) The chips are sprayed or moistened with the pretreating liquor, for example in special apparatus or in the boiler after this has been filled with chips, whereafter the pre-treating liquor is allowed to flow off.

(d) The pre-treating liquor is separated from the chips after the pre-treatment and used anew, possibly after addition of fresh quantity of pre-treating liquor.

The by-products in the sulphate cellulose manufacture can, according to the invention, be refined more easily than what now is the case (no malodorous compounds are produced).

The weight-quantity of sodium-lye (NaOH), required in the pre-treatment, is approximately A of the quantity of NaOH consumed in the sulphate digestion process ac cording to usual methods. The pre-treatrnent according to the invention, however, does not entail any increase of the alkali consumption, because hereof is used for the pre-treatment and A for the digestion as such.

It is preferred to supply approximately of the sodium-lye to the chips in rather pure state (particularly free from Na S). This can be arranged in any of the following ways:

(1) By chlorine-alkali electrolysis of common salt, one can in known manner obtain pure sodium-lye for the pretreatment of the chips, and the chlorine can be used for bleaching the cellulose produced.

(2) In the sulphate cellulose plants there is obtained with the present regeneration systems (recovery of alkali) a melt-soda, substantially consisting of sodium carbonate and sodium sulphide, the relative quantities of which approximately are 4 to 5:1. The present method is to allow the entire melt to flow down into a water quantity suflicient to dissolve the entire quantity of melt, whereafter the solution is causticized with burnt lime in order to transfer Na CO to NaOH. The melt-soda can be mixed with a small quantity of water, so that part of Na CO will remain undissolved. (At 75 C. the solubility of Na CO +Na S is 31 g. per 100 g. solution). It is thus easy to dissolve all Na S while still leaving more than half the quantity Na CO undissolved. This dissolving process can easily be carried out continuously with water flowing in counter-current to the melt-soda. In order to purify the undissolved Na CO further, the latter can be dissolved in pure water to a concentration of 28-30 g./ 100 g. solution. At 30 C. the solubility of Na CO in water is 29 g. per 100 g. solution. If this solution is cooled, the Na CO .10H O crystallizes (=crystallized sodium carbonate), which can easily be carried out in a crystallizator. If hereby the temperature is reduced to 20 C., approximately 75% of that soda quantity dissolved at 30 C. will crystallize. The range of existence for Na CO .10H O lies between 2 C. and +32 C. The crystals can easily be separated from the mother-lye (possibly in connection with washing), be dissolved in pure water as well as causticized, whereby pure NaOH-solution is obtained for the pre-treatment of the wood (the chips). The mother-lye obtained as well as washing solution from the crystal-production are returned to the solution of the melt-soda.

The apparatus, in which the pre-treatment takes place, can comprise a special vessel for this operation with or without agitating and heating means or the pre-treatment can be carried out in the sulphate boiler as a special treatment immediately before the sulphate-digestion proper. In this case the pre-treating liquor is pumped in circulation by means of a circulation pump in usual manner. Preferably one or a plurality of containers for the pre-treating liquor can be connected to the pump. The special vessel may also in some cases consist of a discontinuous boiler in which the pre-treatment solution is separated before the sulphate boiling starts.

In treating the chips with the alkaline liquor, part of the alkali will be consumed, the alkali contents being reduced from the original concentration to a final concentration. In a known manner alkali can be supplied to the solution during the pro-treatment. Further, the solution can in a known manner he used again after adding alkali and likewise, a plurality of solutions can be used in succession with different contents of alkali.

After the pre-treatment has been concluded, at least part of the pre-treating liquor must be removed from the chips, so that digestion liquor for the sulphate digestion (for example a mixture of white lye and black lye) can be filled into the boiler. It is, however, not necessary to withdraw all pre-treating liquor, but the quantity adhering to the chips and absorbed therein may remain. The withdrawn pre-treating liquor can be processed for the extraction of resin and the like.

The best results in carrying out the invention are achieved if the pre-treatment is effected with certain ranges of concentration of the pre-treating solution, certain ranges of temperature and certain ranges for the time of pre-treatment.

Regarding the alkali concentration, tests have thus been carried out with good results with between 5 and 30 g. NaOH per liter for pre-treating 7.5 kg. chips (calculated as dry chips) with approximately 50% water in these chips and 45 liters of pre-treating solution, i.e. there are 600 liters of pre-treating solution per 100 kg. of dry chips, but the most suitable range of alkali concentration lies between 5 and 15 g. NaOH per liter. Below 5 g. the reaction proceeds slowly and is thus time consuming; on the other hand, a quantity of 15 g. sometimes means an unnecessary amount of alkali. Most favorable is 6 to 10 g. NaOH per liter. One can naturally also use a concentration below 5 g. per liter, but then extra large quantities of pre-treating solution will be required. Furthermore, other alkaline substances than NaOH can be used, for example KOH or Na CO The best temperature range for the pre-treatment lies between 50 C. and 70 C., but can be altered both upwardly and downwardly, for example from 30 or 40 C. to 90 C. or above. The lower the temperature, the slower the reaction in the pre-treatment. At temperatures above C., chemical by-reactions can arise. Also, for reasons of heat economy, the range 50 C.70 C. is suitable.

The treatment time has been thoroughly investigated by taking samples of the solution after 5, 10, 15, 20, 30, 40, 50, 60, 70, and 80 minutes during the pre-treatment while the solution is circulating, and these samples have been analyzed with respect to the quantity NaOH, whereupon curves have been plotted on the chemical process in the pre-treatment. It has been found that the reaction time can extend from 5 to 30 minutes, and in certain cases can be limited to 10-15 minutes. Short pre-treatment time is obviously desirable from economic point of view.

It is of particular great value to carry out the pretreatment with the advantage of black lye of different concentrations, but this black lye must be of the kind obtained from a preceding sulphate digestion of chips, previously pre-treated according to the invention. A very great advantage is that pre-treatrnent with black lye goes faster than with sodium-lye only (NaOH in water). The required time when using black lye is approximately half as long as the time for pre-treatment with sodium lye, if the same initial temperature and concentration of NaOH is used at the beginning of the pre-treatment. Thus, the

pre-treatrnent time can be reduced to 58 minutes. The black lye always contains some active alkali, namely between 5 and 8 g. NaOH per liter, which is just suitable for the pre-treatment in question. If one desires to use still higher alkali contents in the black lye, it is merely required to supply the desired additional quantity NaOH to the black lye.

After the pre-treatment, digestion is effected in the usual manner with white lye (NaOH-PNa S), which is mixed with black lye.

The short pre-treatment time when using black lye makes it possible in practice to carry out the pre-treatment in the sulphate boiler proper, without thereby noticeably increasing the circulation time of the boiler. In this case, the black lye is introduced into the boiler simultaneously with the filling of the chips (either from below into the boiler or, for example, sprayed on during the filling of the chips).

The chips in the top part of the boiler, however, would in a discontinuous process require full pre-treatment time, calculated from the time when the filling has been completed. This time can be reduced appreciably by supplying fresh quantity of black lye at the top of the boiler and withdrawing an equally large quantity of black lye from the bottom of the boiler. A sufficient quantity of black lye must then be withdrawn from the boiler to leave space for the quantity of white lye to be added. (This white lye has a rather high temperature.) The withdrawal of black lye and supply of white lye into the digestion boiler can be effected simultaneously if Withdrawal and supply takes place at different levels and the specific weights of the liquors are kept at such values that the white lye is not mixed with the black lye to be discharged. Only thereafter, the circulation and the heat supply are started, and the boiling is performed in the usual manner.

Example l.With NaOH as pretreating liquor (at 50 C.)

Kg. Dry chips 7.5 Water in the wood (:moisture in the chips) +5 5 Liters Pre-treating solution (NaOH solution) 45.0 Water in the wood +5 5 50.5 Withdrawn after finished pre-treatment -39.5

Solution in the chips 11.0 White lye 10.4 Black lye +226 Digestion lye in the sulphate digestion proper 44.0 g. NaOH introduced at pre-treatment 171 NaOH introduced with the black lye 180 NaOH introduced with the white lye (10.4 liters at 147 g. NaOH) 1,529

Example 2.With black lye as pre-treating liquor (at 50 C.)

Kg. Dry Chips 7.5 Water in the Wood +5.4

. Liters Pre-treating solution in the form of black lye 45 Water in the wood +5.4

Withdrawn after finished pre-treatment 37.4

White lye added Q. +11.9

Black lye Q. 10.0 Water +9.1

In the sulphate digestion 44.0 G. NaOH introduced in the pre-treatment 126 NaOH introduced with the black lye 82 NaOH introduced with the white lye (11.9 liters per 140 g.) 1,666

which is 250 g. NaOH per 1 kg. dry chips.

Also in this case there was obtained after the subsequent sulphate digestion (carried out in the usual manner) a cellulose and digestion lye, respectively, free from malodorous substances.

In order to achieve required impregnation of the chips last supplied, that is to say, the chips remaining near the top of the boiler, the boiler can be provided with a lye distributor at the top and a strainer for withdrawal of lye at the bottom, as well as with a circulation system, which permits circulation pumping.

Sometimes it can be expensive to utilize boilers for the pre-treatrnent, if the boiler volume is not already too large for other reasons.

In pre-treatment with black lye, the concentration of the lye can be varied within wide limits, for example from 2 B. to 15 B. (at 60 C. temperature) or still more. The conditions of operating will decide the suitable concentration. Tests have been made with 4 B. and 10 B. with good results.

The black lye penetrates the Wood faster than the NaOH solution does. The organic substances hydrolyze the black lye to NaOH. When, therefore, part of the free NaOH of the black lye has been consumed, new quantities of NaOH will be liberated. This always takes place in the interior of the chip wall, whereby the chemical process will be homogeneous in the entire mass of wood. Therefore, the methyl groups of the entire mass of wood will be converted very rapidly in the pre-treatment with black lye.

What is claimed is:

1. A process of producing sulphate cellulose, comprising pre-treating cellulosic fiber material for a period of 5 to 30 minutes at a temperature between 40 C. and C. with a substantially sulphur and sulphide compounds free solution of alkaline liquor containing between 5 to 30 grams alkali per liter of solution and subsequently digesting the said pre-treated fiber material in a sulphate digestion process.

2. A method according to claim 1, wherein the pretreatment is carried out with a solution containing NaOH.

3. A method according to claim 1, wherein the pretreating liquor is separated from the fiber material prior to the subsequent digestion process.

4. A method according to claim 1, wherein excess of alkali is separated from the fiber material and used in a new pre-treatment process.

5. A method according to claim 1, wherein approximately of the quantity of alkali required for the digestion is used for said pre-treatment of the fiber material.

6. A method according to claim 1, wherein the alkali solution consists of purified and causticized melt-soda obtained from a previous sulphate digestion process.

7. A method according to claim 1, wherein said pretreatment With alkaline liquor is carried out in a separate vessel, before supplying said cellulosic material into a boiler for the sulphate digestion process.

8. A method as claimed in claim 1, wherein said pretreatment at least partly is carried out by means of black 1yeobtained from a preceding sulphate digestion of cellulosic material which has been pre-treated with alkaline liquor simultaneously with filling wooden chips into a boiler for subsequent sulphate digestion.

References Cited UNITED STATES PATENTS 9/1932 Richter 162-82 5/1962 Schnyder 16238 OTHER REFERENCES Wells Kraft Pulping With Low Sulphidity, from Paper Trade Journal, TAPPI sec. pp. 103-106, Aug. 28, 1941.

10 DONALL H. SYLVESTER, Primary Examiner.

H. R. CAINE, Assistant Examiner. 

